Animals with pouches belong to the class of mammals called marsupials. Marsupials include kangaroos, bandicoots, wombats, banded anteaters, koalas, opossums, wallabies, Tasmanian devils, and many other species. The pouch, also called a marsupium, is located on a female marsupial's abdominal area and is part of her reproductive system. A mother marsupial carries her undeveloped offspring in her pouch, following a very short gestation period (gestation is the period of time between fertilization and birth). The newborn marsupial finishes maturing in the mother's pouch.

The reproductive process of marsupials is quite different from that of placental mammals. Placental mammals are animals, including humans, in which the fetus remains in the mother's uterus and receives nourishment from the placenta throughout gestation until birth. A female marsupial, on the other hand, invests relatively few resources during the brief gestation period. She expends more energy in the development of her young during the lactation (nursing) period, when the young are in the marsupium.

Marsupials appear to have a reproductive advantage over placental mammals for the following reason: If the offspring of a marsupial dies during the gestation or lactation stage, the female marsupial can conceive again sooner than a placental mammal in a comparable situation.